Simply ask him and tell him that his boss called. If he denies it. Tell him no sex ever.

Maybe he's ill and getting treatment and did not want you to worry? It happens. He may not be a cheat. Ask him about it

Time to buy a sniper.

As a man, my first though is out and out deception to hide an affair. If it's something else like, say, a sudden and crippling addiction to trainspotting or bird-watching, either way I would confront him and did out. The other choice, if you wish to take the more extreme route, is to hire a PI and have him followed, or put him up for an episode of Cheaters :-D

Of course he is cooking meth.

OP here! Mm so, at least he was upfront with me about it. He hasn't been cheating (thank god)- comment 104 was spot on- He's been finding work stressful, started skipping, and has been spending his time gaming (he's now handed his steam account over to me). He was too embarrassed/ashamed to tell me how hard he was finding it. He went and begged his boss for his job back today, which (to our shared surprise) was granted. She was sympathetic about him finding the work stressful since he has an anxiety disorder. The lying is not going to be easy to get past- in the ~16 months we have been together he has never done ANYTHING like this, so we are going to give it another shot. In the meantime he is going to be grovelling, at least he seems genuinely remorseful, and knows that no such behaviour will be tolerated again.

You may want to consider contacting Joey Greco. :-/

Maybe he is out looking to buy you a ring....

tell him you stopped by his work too see him and see how he reacts

Sounds like Walter in an episode of breaking bad :P